Inorg. Chem. 1985,24, 1906-1911
1906
of Naval Research, Washington, DC, for a joint grant in colcomplexes show visible-region CT spectra typical of the ruthelaboration with Prof. A. J. Bard. nium(I1) component. In the solid state the complexes are intensely colored, have FTIR v(CN) frequencies differing from the simple Registry No. R~(bpz)~~', 75523-96-5; Fe(H20)s3', 15377-81-8;Fealkali-metal hexacyano anions, and presumably do involve some (H20)2', 15365-81-8; C U ( H ~ O ) ~22174-1 ~ + , 1-4; [Fe(CN),I4-, 13408charge transfer under these conditions. A strong solution of the 63-4; [Fe(CN)6l3-, 13408-62-3; [C0(NH3)6I3', 14695-95-5; [CO(NH3)sC1]2', 14970-14-0; C O ( H ~ ~ )15276-47-8; ~~+, Mn(Hzo)6*+, ferricyanide ion pair in aqueous solution shows a broad band 15365-82-9;R~(bpz)2(bp~H)~', 95865-85-3;R u ( ~ P z H ~ )95865-86-4; ~', centered about 15 150 (670)cm-'. This is not present in either 776 1-88-8; I-, 2046 1-54-5; N,N'-diphenyl-p-phenylenediof the components and may be an intervalence t r a n ~ i t i o n . ~ ~ - ~Ag'/N03-, ~ amine, 74-3 1-7; phenothiazine,92-84-2; N,N-dimethyl-p-toluidine, 99Acknowledgment. The authors are indebted to the Natural 97-8; N,N-dimethylaniline,121-69-7; diphenylamine, 122-39-4; aniline, Sciences and Engineering Research Council of Canada for fi62-53-3;triphenylamine,603-34-9; 1,2,44methoxybenzene, 135-77-3; nancial assistance through operating and strategic grants and for 1,4-dimethoxybenzene, 150-78-7; 1,2,3-trimethoxybenzene,634-36-6; 1,2-dimethoxybenzene, 91-16-7; 1,3,5-trimethoxybenzene,621-23-8. a Summer Studentship to G.E. We are also indebted to the Office
Contribution from the Department of Chemistry, York University, Downsview, Toronto, Ontario, Canada M3J 1P3
Protonation and Lewis Acid-Lewis Base Equilibria in (Bipyrazine)molybdenum and (Bipyrazine)tungsten Tetracarbonyls ELAINE S.DODSWORTH, A. B. P. LEVER,* GORAN ERYAVEC, and ROBERT J. CRUTCHLEY
Received July 30, I984 The title complexes react with boron trifluoride etherate to generate mono-and diadducts of BF3. In H2S04/ethanolsolution one proton is coordinated. In each case reaction is assumed to occur at the peripheral uncoordinated nitrogen atoms of the bipyrazine unit. New metal-to-ligandcharge-transfer bands are observed for these various species. Analysis of the spectra shows that pK,(I) for the first uncoordinated nitrogen atom is about 4 . 3 (Mo complex) and that extensive mixing of ground and excited states must be occurring to account for the oscillator strengths and band widths observed.
Introduction Ground- and excited-state protonation equilibria involving the = bipyrazine) have recently been reported.' R ~ ( b p z ) ion ~ ~ (bpz + This species binds up to six protons in a stepwise fashion, providing an interesting series of electronic absorption and emission data. Of special interest was the variation in metal-to-ligand chargetransfer (MLCT) energy as a function of the degree and site of protonation. Stepwise protonation provides a useful mechanism for "tuning" excited-state redox potentialsZ and is of obvious interest in the design of photocatalytic redox reagents. Previous studies of protonation equilibria involving inorganic complexes have discussed protonation at the nitrogen atom of coordinated cyanide ion in species such as M(CN)4L and M(CN)zL2 (M = Fe, Ru; L = d i i m i ~ ~ e )considered ,~.~ the enhanced acidity of ruthenium(I1) complexes of 4,7dihydroxy-1,lO-phenanthr~line~, and analyzed the pH dependence of ruthenium bipyridine6 and bipyrimidine' species. As is evident, much of the work has been associated with ruthenium or its congeners. The binding of Lewis acids to cyanide complexes and its effect on their charge-transfer spectra has also been studied.* For these reasons we considered it useful to probe the protonation equilibria in a complex other than ruthenium having only one bipyrazine unit to provide a data set that might be capable of more detailed analysis and additional insights. The species M(CO),(bpz) (M = Mo, W)9 are soluble in organic solvents and give rise to intense absorption in the visible region, Crutchley, R. J.; Kress, N.; Lever, A. B. P. J. Am. Chem. SOC.1983, 105, 1170. Haga, M.-A.; Dodsworth, E. S.; Eryavec, G.; Seymour, P.; Lever, A. B. P. Inorg. Chem., paper in this issue. Schilt, A. A. J . Am. Chem. SOC.1963,85, 904. Peterson, S. H.;Demas, J. N. J. Am. Chem. SOC.1979, 101, 6571. Giordano, P. J.; Bock, C. R.;Wrighton, M.S. J. Am. Chem. Soc. 1978, 100, 6960. Giordano, P. J.; Bock, C. R.; Wrighton, M. S.; Interrante, L. V.; Williams, R. F. X . J . Am. Chem. SOC.1977, 99, 3187. Hunziger, M.;Ludi, A. J. Am. Chem. SOC.1977, 99, 7370. Shriver, D. F.; Posner, J. J. Am. Chem. SOC.1966, 88, 1672. Crutchley, R. J.; Lever, A. B. P. Inorg. Chem. 1982, 21, 2216.
Table I. Electronic Spectra of M(CO)4bpzin the Presence of Et,O.BF," A,,
approx concn of Et,O.BF?, mol
L-1-
0
0.07 0.13 0.17 0.20 0.33 7.96
Mo(C0)4bpz color I1 I/Ib Ia pink 368.5 520 pink 369 523 sh 527 618 mauve 370 blue 371 sh 625 371 494 640 blue 490 653 gray-blue sh gray-blue ~
nm
W(C0)4bpz Ia
I1 I/Ib 371 533
371 534 371 539 371 sh sh 482 ... 480 322 490
sh 616 620 630 640 654
OSpectra recorded using approximately M M(C0)4bpz in acetone. Solutions were deoxygenated with dry N2 before addition of Et20-BF3.bComplexdissolved in pure Et20.BF3.
-
attributed to metal-to-ligand charge-transfer (MLCT) M bpz(a*). Addition of mineral acid, or the Lewis acid BF3 (etherate) causes changes in the visible absorption spectra that can be interpreted in terms of mono- and diacid equilibria. FTIR and NMR data are reported in support of the equilibria proposed.
Experimental Section The complexes M~(Co)~(bpz) and W(CO)4(bpz) were prepared according to literature method^.^ Acetone and 96% H2S04were BDH Analar grade. The acid was diluted with absolute ethanol. Boron trifluoride etherate was purified according to a literature methodlo and stored under nitrogen or dry air. Electronic spectra were recorded on a Perkin-Elmer Hitachi Model 340 microprocessor spectrophotometer. The cell holder was cooled to ca. 10 "C to minimize decomposition of the complexes. 'H NMR spectra were recorded on a Varian EM360 60 MHz spectrometer at ambient temperature. Tetramethylsilaneat 0.00 ppm or the residual protons of acetone-d6 at 2.05 ppm were used as internal references. FTIR spectra were recorded on a Nicolet SX20 instrument (courtesy of Nicolet Co.) as acetone solutions in a sodium chloride cell. Computer simulations were obtained with a Commodore (10) Perrin, D. D.; Armarego, W. L. F.; Perrin, D. R. 'Purification of Laboratory Chemicals", 2nd ed.;Pergamon Press: Elmsford, NY, 1980.
0020-1669/85/1324-1906$01.50/0 0 1985 American Chemical Society
Inorganic Chemistry, Vol. 24, No. 12, I985 1907
Protonation in (Mo/W)(bpz)(CO), Table 11. Spectroscopic Data in Ethanol/H2S04' band I1 concn of H2SO4, mol L-l h" nm
band I/Ib c
, ,A
nm
(a) Mo(CO)&pz 0 0.5 1.o 1.5 2.0 2.5 3.0 3.5 4.0
377 377 377 377 377 377 377 376 (sh) 376 (sh)
6180 5880 5350 5270 4800 4520 3800 3420 2660
0 0.5 1.o 1.5 2.0 2.5 3.0 3.5 4.0
380 379 379 379 379 380 380 (sh)
6670 6470 5980 4870 4690 4320 3510
547 542 542 542 546 548 (sh) 540 (sh) 522 522
(b) W(C014bPZ 558 554 556 557 560 (sh) 520, 540 (sh) 502, 520 (sh) 506 503
band Ia , ,X
c
7430 6870 6080 5610 5080 4850 4800 5980 7410
nm
c
sh 630 634 637 640 642 647 658
9280 8520 7650 6700 6230 5720, 6180 6960,6910 9370 10510
3340 4950 6560 7680 8860 9700 9630
sh 628 63 1 633 634 636 640 644
6 600 8 590 10 280 12320 12620 12910 11 680
e = molar absorbance (L mol-l cm-I); sh = shoulder. Molar absorbances are not corrected for decomposition. Spectra were recorded immediately after mixing and errors are 4 M (Mo) and >3.25 M (W) H2S04. When acid solutions ([H’] C 1 M) are diluted or neutralized, bands I and I1 reappear and band Ia diminishes. However, if strong acid solutions, in which band l b is present, are diluted or neutralized, the reversal is incomplete and the spectra are broad in the region of band I. Furthermore, if a solution in fairly strong acid is monitored with time, band Ib increases slightly over short time intervals (minutes) as band Ia decreases. This increase is not associated with growth in band 11and cannot therefore reflect an increase in the concentration of the parent complex. Over longer time periods both bands Ia and Ib diminish. We return to this problem below. Tetracarbonyl diimine complexes of C , symmetry have four allowed CO stretching modes in their IR spectra: AI, B1, A,, B,. In Mo(CO),bpz the two higher frequency bands correspond primarily to the trans carbonyls and the two lower bands primarily to the cis ~ a r b o n y l s . ~ J ~ FTIR J* spectra of the C O stretching region and corresponding electronic spectra were recorded for solutions of Mo(CO),bpz in acetone and in the presence of varying amounts of Et20.BF3. Apart from some broadening of the CO bands as BF, was added, there was no significant change in the
-
-
-
(17) Adam, D. M.“Metal-Ligand and Related Vibrations”; Edward Arnold: London, 1967; p 101. (18) Kraihanzel, C. S.; Cotton, F. A. Inorg. Chem. 1963, 2, 5 3 3 . Cotton, F. A,; Kraihanzel, C. S. J . Am. Chem. SOC.1962, 84, 4432.
Inorganic Chemistry, Vol. 24, No. 12, I985 1909
Protonation in (Mo/W)(bpz)(CO), Table IV. ‘H NMR Spectra of Mo(CO),bpz and W(C0)4bpz4
Mo(C0)pbpz +Et20.BF3 (excess) W(C0)4bPz +Et20.BF3 (excess)
H3
H5
H6
10.05 10.42 10.08 10.41
8.90 8.91 8.85 8.80
9.27 10.14 9.35 10.19
“Data in ppm from Me4%;recorded in acetone-& spectrum during the growth of band Ia. When there was added a sufficient excess of Et20.BF3 corresponding to the appearance of band Ib in the electronic spectrum, the energies of all four v(C0) increased (Table 111). Assignments of the parent carbonyl spectrum (Table 111) have already been d i s c u ~ s e d . ~As expected for the CO band force constants, k l (CO groups trans to diimine) < k2 (CO groups perpendicular to diimine plane). We anticipate that boron trifluoridation will increase the acceptor power of the diimine and lead to an increase in both k , and k2. The assignment shown in Table I11 for the BF, adduct fulfills this expectation, though the fit to the Kraihanzel-Cotton matrices’* is not quite as good as for the parent species. This poorer fit probably reflects the fact that the dominant species in solution is the monoadduct, which possesses at best C, symmetry, rather than the C2, symmetry assumed by the theoretical analysis. Other assignments provide even poorer fits to the matrices or result in k , > k2. This would imply that bpzBF, is a better acceptor than CO, which is an improbable result. Note that these data permit one to derive the ligand effect constants defined by Timney for deducing carbon monoxide stretching force constants.19 The bpz and BF, adduct may be compared with bipyridine and CO, as follows: cis const, N m-’ bPY bPZ bpzBF,
co
trans const, N m-’
-24
-62
-13.5 -5
-5 9 -52.5
33.5
126.1
The variation is consistent with the ligands becoming better a acceptors in the sequence bpy C bpz < bpzBF,
